The present invention relates to a deflection control type ink jet recording apparatus which is capable of automatically removing a failure in ink ejection.
There is known an ink jet recording apparatus of the type which ejects ink from a nozzle while applying vibration thereto, causes a charging electrode to selectively charge ink drops in a position where the ink from the nozzle separates into a drop, and allows deflection electrodes to deflect the charged ink drops toward predetermined positions on a sheet.
In an ink jet recorder of the type described, the ink continuously ejected from the nozzle separates into a string of drops having a predetermined size in the course of its flight away from the nozzle. The separation is attained by applying high frequency vibrations to ink fed under pressure to an ink ejection head. Means for applying such vibrations to ink generally comprises a flat or cylindrical electrostrictive vibrator. The ink drops are selectively charged in response to input video data and then passed through a predetermined deflecting electric field. This brings about a deflection in the path of each drop which is determined by the presence/absence of a charge thereon or a specific level of a charge. As a result, the drops impinge on certain positions on a sheet or collected by a gutter in accordance with the video data, thereby reproducing a desired image on the sheet. As well known in the art, charging in the art of ink jet recording utilizes the fact that ink breaks into a drop while maintaining at its front end a charge which has been developed by electrostatic induction due to the electric field developed by the charging electrode, the charge remaining on the separated ink drop.
The sheet is usually located at a relatively long distance from the ink ejection nozzle. Therefore, the ink pressure is designed high enough for an ink drop to stably fly as far as the sheet despite the influence of the charging and deflecting electric fields. Stable and accurate control over the ink viscosity and pressure, vibration pressure, charge amount, deflection electric field and the like is another important consideration in regularly forming drops of a predetermined size and causing them to accurately fly predetermined deflection paths. Further, adequate charging of drops is unattainable unless a charge voltage or pulse is applied precisely timed to the separation of a drop from the ink.
In light of the above, there has been employed a phase search before a record charge control in order to stabilize the ink to a predetermined pressure and a predetermined viscosity while predetermining a timing for the application of charge pulses. For the phase search, a contact or non-contact type charge detector electrode is connected to a charge detector circuit whose major components are an amplifier, an integrator and a comparator. A charging pulse having a short duration is applied to the charging electrode, and the phase of the charge pulse relative to the separation of an ink drop is sequentially shifted. When the charge detector circuit produces an output indicating "charged", the phase of the then appeared charge pulse is predetermined to be the adequate charging phase. Thereafter, the deflection is adjusted if necessary and a printout operation is initiated.
Now, the ink jet recorder discribed above is unable to operate in the expected manner if any air bubble or dust is present in the head at the initial stage of ink ejection. The amount of air bubbles in the ink depends a great deal on the ink supply cut-off time in the previous interruption of ink ejection or the time for which the recorder has been left unused. In such a situation, a proper charging phase and/or deflection amount cannot be preset resulting in error in the phase search and/or deflection setting. In response to the error, the ink ejection is suspended and the whole printer is maintained inoperable until the operator removes the faulty condition. Thus, the prior art ink jet printer is totally disabled if the ink ejection is inadequate at the sacrifice of running efficiency and labor required for maintenance.